DESCRIPTION: (Adapted from the applicant's Description) The focus of this revised application is to determine if a correlation exists between myelination and neurotransmitter production in phenylketonuria. The investigator's preliminary data show that myelination, demyelination, and remyelination take place in PKU mice as they are cycled on rounds of low Phenylalanine diet and regular diet. Since blood Phe levels change rapidly compared to changes in myelination, she proposed to examine the relationship between gain and loss of myelin and neurotransmitter production. The investigator will employ histochemical, immunohistochemical, molecular, and biochemical techniques to better understand the pathophysiology of PKU, and ultimately lead to the development of additional therapies for PKU.